The present invention relates to a dot matrix printing head of the type in which the printing needles are guided for movement by the matrix through two adjacent parallel rows of holes therein, each lying in a plane parallel to the printing plane, and arranged to effect high-speed printing o high-definition printing.
Dot matrix printing heads are known, particularly with 24 needles, which are arranged to effect high-speed, low-definition, or "draft", printing and high-definition, or NLQ (near letter quality), printing at a lower speed in alternation, which, although still being distinguished, have a guide matrix for the printing needles provided with a double row of holes disposed side by side so that the tips of the needles which are intended to impact on the printing ribbon to leave a corresponding ink print in the form of a dot on the sheet to be printed, are disposed in two adjacent, parallel rows; the print characters are formed by the printing of a plurality of dots on the sheet in predetermined grids (matrices) by the selective operation of the printing needles. In one known embodiment, the support and guide matrix for the needles is formed from two elements which are relatively slidable parallel to the two rows of needles, one row being guided on a first of the matrix elements and the other on the second; thus, during high-speed, low-definition printing, the needles in the two rows are kept perfectly together in adjacent, coplanar pairs; during high-definition printing (at lower speed), on the other hand, the two elements of the matrix are moved vertically so that the needles in one row are staggered relative to those in the other so that the two rows of needles print characters with partially-superposed dots. This system works quite well but has the disadvantage of requiring complicated mechanisms to effect the movement of the two matrix elements; these mechanisms, in fact, are subject to considerable stress in use and therefore must be adequately dimensioned. This complexity on the one hand reduces the overall reliability of the system and on the other hand renders the drive for the translational movement relatively slow.